U.S. Pat. No. 4,760,421 issued July 26, 1988 discloses a printer using a CRT, or CRT equivalent, and a fiber optic bundle. The bundle has first and second faces in which the light-guiding fibers are aligned in a linear array and in an area array respectively. The linear face abuts an electrostatic (electro-photographic) medium and the face plate of the CRT is imaged to the area face of the bundle. The printer includes a permanent memory which stores the digital addresses of electron beam positions on the face plate each of which corresponds to a specific pixel position in the linear face. The beam of the CRT is directed to consecutive ones of those addresses. The phosphor excited into luminescence by the impinging electrons on the face plate, in response, introduces light into a sequence of fibers in the area face. The fibers guide the light to the sequence of pixel positions in the linear face. The light exiting the linear face is operative to selectively discharge the electrostatic medium, pixel by pixel in a manner to create a line image of surface charge or corresponding electrostatic potential variation. The electrostatic medium moves at constant speed or step by step so that the succession of line images produces an area image in a raster fashion. The area image on the electrostatic medium is used to create an image, for example, on plain paper as in the familiar xerographic process.
The sequence of digital addresses for the beam on the face plate and the associated pixel in the linear face is obtained electronically by an initialization procedure which, for example, employs the CRT to move an electron beam in sequential fashion to each of predefined addresses in a dense grid, say 512.times.512, of positions on the face plate during each complete scan of the face plate. The phosphor responds to generate light at different addresses of the linear face corresponding to the beam addresses. During each complete scan of the face plate, a photodetector, apertured through a narrow slit and moved incremently along the linear face, senses an output only when the face plate address at which light is generated coincides with a faceplate address position that produces light which is focussed or transmitted into the fiber at which the photodetector is positioned. Generally, there will be many electron beam addresses in a cluster at which light will be transmitted into the fiber. Only one of these need be chosen at the center of the cluster. The initialization procedure is continued until the detector has sensed the presence of light at each pixel position in the linear face and those positions are associated with corresponding optimum face plate addresses of the electron beam.
The sequence of electron beam addresses and the corresponding pixel positions in the linear face established during the initialization procedure is placed in a look up table. The look up table is used, for example, to burn a programmable read only memory (PROM) which is used during normal operation to control the succession of beam positions during production of each linear (line) segment of the image being exposed.
The patent discloses that a magnetic deflection CRT or an electrostatic CRT can be used for such a printer. The scan trajectory described in the patent can be described as a vector scan sequence chosen such that light emerges in a progressive sequence from the fibers in the linear face. The electrostatic CRT is useful in the vector scan mode because changing electric fields rapidly is easier than changing magnetic fields. So high speed or lower power operation is attainable. Although the electrostatic CRT is faster than the magnetic CRT, it is more costly and has other disadvantages. The present invention is directed at minimizing the total trajectory of beam excursions during accessing of the sequence of addresses, thereby mitigating the requirement for high speed operation, thus making the lower cost magnetic CRT more attractive.